The ultrasonic transducer is a device for performing the radiation and reception of sound waves beyond the audible range (about 20 Hz to 20 kHz) and is widely used in medical applications, non-destructive inspection, and the like. Currently, the most extensively used as the ultrasonic transducer are piezoelectric devices typified by PZT (Lead Zirconate Titanate). However, in recent years, ultrasonic devices called Capacitive Micro-machined Ultrasonic Transducers (hereafter, referred as CMUTs), which are based on an operation principle different from that of piezoelectric type devices, have been developed and are being put into practical uses. The CMUT are fabricated by applying semiconductor technology. In general, they are formed by embedding an electrode material into a substrate made up of a member that is used in semiconductor process, such as silicon (the substrate itself may provide the electrode), and securing a minute (for example, 50 μm) and thin (for example, several μm) diaphragm with supporting posts surrounding the diaphragm. A cavity is provided between the diaphragm and the substrate such that the diaphragm can vibrate. An electrode material is also embedded in the diaphragm. Thus, disposing separate electrodes in the substrate and the diaphragm allows the structure to function as a capacitor. Further, applying voltage to both the electrodes (generally, a bias voltage is applied in advance) makes it operate as an ultrasonic transducer. Applying alternating-current voltage (AC voltage) to both the electrodes varies the electrostatic force between the electrodes, thereby causing the diaphragm to vibrate. If, at this moment, any medium is present in contact with the diaphragm, the vibration of the diaphragm propagates in the medium as a sound wave. That is, it is possible to radiate sound. Conversely, if a sound wave is transmitted to the diaphragm, the diaphragm vibrates in response thereto, thereby changing the distance between both the electrodes so that an electric signal flows between both the electrodes. Thus, it is possible to receive the sound wave by taking out the electric signal.
The diaphragm of CMUT is dynamically connected to the underlying substrate via the supporting posts. Therefore, when the diaphragm vibrates, the vibration propagates not only to the medium but also to the substrate. Alternatively, the electrostatic force generated between the electrodes of diaphragm side and substrate side acts equally on both the diaphragm and the substrate. Thus, electric vibration propagates to the substrate through electrostatic force. In this way, in a CMUT, vibration will propagate not only to the diaphragm, but also to the substrate by way of dynamic or electrical action. This vibration propagating through the substrate is reflected from the substrate toward the diaphragm side, and is detected again as an electric signal. These signals will become undesired response in performing normal ultrasonic transmission/reception. These undesired responses will become artifacts in an ultrasonic imaging device for medical use and a non-destructive inspection apparatus, and increase the risk of erroneously evaluating diagnoses and inspection results. Therefore, in using the CMUT, suppression of signal components of the vibration through the substrate will become extremely important. Patent Literature 1 describes that the effect of artifact signals is avoided by reducing the substrate thickness to not more than a certain thickness so that frequency components of the signal which can provide artifact components are brought out from the sensitive band of the transducer. Moreover, Patent Literature 2 describes a technique to avoid the directivity of ultrasonic wave from deteriorating due to a lateral wave excited in the substrate by optimizing the substrate thickness and providing a slot and porous in the substrate.